Anti-idiotypic antibodies (Ab2) are directed against the antigen binding site of other antibodies (Ab1). Epitopes of diverse pathogenic antigens may be mimicked functionally and structurally by those Ab2s. Thus, Ab2s are able to induce anti-anti-idiotypic antibodies (Ab3) that are Ab1-like and have similar biological properties [1].
Different kinds of Ab2s are distinguished after immunization with Ab1 [2, 3]. The Ab2α (Ab2-alpha) recognizes idiotypes at the distal position from Ab1 paratope and does not interfere with Ab1 binding to the nominal epitope. Ab2s beta (Ab2β) are directed to the paratope of Ab1s and bear the internal image of the antigen. Ab2β can compete with the nominal antigen for binding to the idiotype and to antigen-specific xenogeneic antibodies. A third category, termed Ab2γ (Ab2-gamma) recognizes idiotopes physically close to the binding site but does not behave as a surrogate antigen [4].
Moreover, internal image anti-idiotype antibodies may exhibit identical three-dimensional conformation or amino-acid sequence homology with the epitope. This has been shown for the reovirus system, where the amino acid sequence of the idiotype had the same motif as the virus hemagglutinin [5]. Thus the virus-receptor binding on susceptible cells can be prevented.
Generally, the advantages of anti-idiotypic antibodies over other vaccines applying peptide antigens or inactivated viruses can be manifold [6]. They do not involve any viral components, they are safe but do not miss any specificity by their mimicry of single epitopes. They can be produced easily in large quantities for immunization. Ab2s can even induce an immune response against single neutralizing epitopes that may not be immunogenic on the native antigen. They thus can break immunogenic unresponsiveness to certain antigens as has been demonstrated in neonatal mice [7]. Active immunotherapy with Ab2-based vaccines have proven very potent to induce antigen-specific Ab3 in cancer patients against tumor cells [8-11].
Ab2s that display internal images of antigen (Ab2β) have also induced protective immunity against infective pathogens [12].
HIV-1 infections may be accessible to Ab2 therapy because of the importance of neutralizing antibodies in the control of HIV-1 infections [13]. The possibility to induce neutralizing Ab3 in various systems including HIV-1 has been described [14].
Cross-reactivity of HIV-1 antigens with various normal human antigens suggests a potential advantage of single epitope Ab2 vaccines over multiple epitope vaccines [15]. Different approaches for the induction of a neutralizing humoral immune response have been made with anti-idiotypic antibodies. The most promising experiments were undertaken with anti-CD4 specific mAbs, which are able to induce HIV-1 neutralizing antibodies in vitro [16, 17]. The anti-CD4 mAbs Leu3a and IOT4A were also tested in clinical studies [18, 19] and were-shown to induce a gp120 cross-reacting immune response that inhibits gp120 binding to CD4. Ab2s against V3 loop peptides were also generated and used in different in vitro studies [20-22]. In different animal models they induced Ab3s with the ability of binding the autologous peptides as well as gp120, but could not neutralize HIV-1 in an in vitro experiment. Only one Ab2 has been described having the potential as a vaccine against HIV-1 [23].
In principle, Ab2 beta antibodies raised against antibodies neutralizing HIV-1 might have an enormous potential for vaccine design. A vaccine against HIV-1 must induce both, cellular as well as humoral immune responses. HIV-1 specific CTL-responses can be induced by various experimental vaccines such as DNA-vaccines [24] or chimeric influenza live viruses administered as nasal spray and expressing HIV-1 epitopes [25]. Reasonable experimental vaccines inducing neutralizing antibody responses are so far generally not available. The human monoclonal antibody 2F5 (produced by hybridoma cell line ECACC Accession No. 90091704) broadly and potently neutralizes primary HIV-1 isolates [26]. Thus, mAb 2F5 is an extraordinary interesting tool for the design of an anti-HIV-1 vaccine leading to properly induced humoral immune responses. It recognizes the highly conserved six amino acid core epitope sequence ELDKWA (SEQ ID NO:1) on the virus envelope ectodomain gp41 [27]. 2F5-like specific antibodies are only rarely found in sera of HIV-1 infected individuals. HIVIG (pooled sera of more than 70 HIV-1 positive donators) do not show significant 2F5-like specific binding to gp160 and/or gp41. The region on gp41 to which 2F5 binds is obviously cryptic to the human immune system during natural infection. Different mechanisms might contribute to its immuno-cryptic behavior. One explanation might be that the human complement factor H which is abundantly present in normal human serum has a binding domain in that region of gp41 thus masking it against immune recognition [28]. This mAb, although being a broad and potent entry blocker (neutralizers) of HIV-1 binds only very weakly to free virus and HIV-1 infected cells. [30]. It is therefore concluded that this fusogenic region on gp41 is only exposed during the event of fusion of the HIV-1 with the host cell. That might on the one hand give an additional explanation for the cryptic nature of these neutralizing epitopes during natural infection. On the other hand HIV-1 isolates that are neutralization insensitive to this antibody have not been found so far.
In earlier studies the core epitope motif ELDKWA (SEQ. ID NO: 1) of 2F5 was integrated into different antigenic formats. The presentation of the ELDKWA-motif (SEQ ID NO: 1) on the haemagglutinin of influenza life virus was able to induce long lasting mucosal immune responses detected as 2F5-like specific IgA's in mouse faeces upon repeated immunization as a mucosal nasal spray [25]. However, quantities of IgA sufficient to prove in vitro neutralization potency could not be extracted from the faeces samples. Other forms of presenting the 2F5-core epitope for immunization such as fusion to the hepatitis B surface antigen expressed in yeast induced very high 2F5-like specific ELISA-titers in immunized BALB/c mice. However, the sera of those animals did not show any significant in vitro neutralization potency [36]. Peptide versions of the 2F5 epitope were also poorly immunogenic.